Presently available techniques for atomizing and applying coating materials to surfaces of products include discharging liquids through small apertures under high applied pressure, introducing the liquid to the center of a high speed rotating disk, introducing the liquid into a high velocity stream of air, introducing a liquid jet or film to an intense electrical field and introducing the liquid to a surface which is caused to vibrate at an ultrasonic frequency. The advantages and disadvantages of the various known implementations of these atomizing techniques are extensively documented in technical journals and texts. For example, a comprehensive technical survey of the known methods is set forth in "Atomization and Sprays," by Arthur J. Lefebvre, Purdue University, Hemisphere Publishing Corporation, 1989.
Ultrasonic liquid atomizing spray systems have generated considerable attention as evidenced by prior U.S. patents. It is known in the prior art that a film of liquid on a surface can be converted into a mist of small drops by vibrating the surface at an ultrasonic rate. Also, prior art teaches that the size of the drops in the mist are related to the rate of vibration. However, problems associated with introducing liquid to a vibrating surface in a manner to produce dependable, uniform spray patterns have significantly limited the effectiveness and therefore the commercial acceptance of prior art approaches. Also, problems with controlling the precise amplitude of the vibrations in the various sections of the surface significantly influence the characteristics of the produced spray and affect the quality of an applied coating.
In known ultrasonic spray coating systems, the coating material is first disintegrated into a fog of tiny droplets which is injected into a laminar gas stream to create a laminar material spray. The spray is directed at an item to be coated. The flow rate of material being disintegrated is regulated to control the volume of material injected into the gas stream thereby controlling the volume of material applied to the item and, hence, the concentration of solids which remain after coating.
The known method of coating is very expensive and difficult to undertake. Furthermore, it is inefficient, because it coats everything in the area of the item, as well as the item. The prior art approaches have failed to provide adequate means to achieve spray patterns which produce coatings of desired uniformity and definition. There is a great commercial need for improved techniques and systems for applying liquid coating material to surfaces such as printed circuit boards, semiconductor wafers, continuous sheets of float glass, automobile trim, continuous sheets of woven and non-woven materials, etc., with improved precision, efficiency and rapidity.
Ultrasonic liquid atomizing spray systems have generated considerable attention. It is shown in the prior art that a film of liquid on a surface can be converted into small drops by vibrating the surface at an ultrasonic rate. Prior art teaches that the size of the drops are a function of the vibration frequency and amplitude. Also, prior art shows many ways of introducing the liquid to a vibrating surface. However, problems associated with introducing a sufficient flow of liquid to an ultrasonically vibrating surface in a manner to produce dependable, uniform spray patterns have significantly limited the effectiveness and therefore the commercial acceptance of prior art approaches. Additionally, problems with controlling the flow of ultrasonic energy into the atomizing liquid significantly influence the characteristics of the produced spray and the resultant quality of an applied coating.
Prior art approaches generally describe various cylindrical, nozzle-shaped ultrasonic structures, with the liquid spray material being introduced in the center of the nozzle spray forming tip and also occupying a portion of the path of ultrasonic energy propagation. The basic difficulties with these approaches are that considerable ultrasonic energy is lost to the liquid supply connections and to the liquid within the structure, and the spray patterns produced by such structures are cylindrical thereby coating thickness distributions on surfaces tend towards a gaussian rather than a uniform shape.
Thus, most, if not all, prior atomizers produce questionable or unsatisfactory shape and uniformity characteristics for precision coating applications. Significant commercial potential therefore exists for a system which forms an ultrasonically atomized mist of fine droplets from a coating of liquid in a spray having a predetermined (but controllable) pattern, uniformity and velocity such that deposition of a precision shape and uniformity may be made on an object surface to be coated with a minimum loss of the coating liquid to the environment or to unwanted surfaces. The present invention provides such a system.